In today's digital subscriber line (DSL) deployments, different variants of DSL techniques are used. For example, DSL techniques providing a rather low bit rate such as ADSLx; e.g. ADSL, ADSL2, ADSL2+ or READSL; and other DSL techniques that provide a comparatively high bit rate such as Very High Speed Digital Subscriber Line (VDSL); e.g. VDSL1 or VDSL2; are often used on different telecommunication lines that are grouped into a single binder arranged between a building and a cabinet of the network operator. Telecommunication lines configured for rather low bit rate communication typically end at DSL modem circuitry installed at a central office or a local exchange of the network operator. For these low bit rate telecommunication lines, the cabinet mainly serves as a cross-connect for electrically connecting the wires between the central office or local exchange and the cabinet to wires between the cabinet and the individual buildings.
Because high bit rate DSL techniques require the telecommunication line between the network operator's modem circuitry and a mode circuitry of the customer's premises equipment (CPE) not to exceed a maximum length of typically about a few hundred meters, the operator's modem circuitry of these high bit rate telecommunication lines is often installed in the cabinet. In other words, the cabinet may include an access node for the high bit rate communication lines. The access node may be connected via a high bit rate backhaul link, such as an optical link, to a communication network of the operator (Fiber To The Cabinet, FTTC). A cabinet having such an access node is also referred to as remote unit or flexibility point.
In order to avoid excessive crosstalk from the high bit rate lines to the low bit rate lines in a section of the low bit rate line in which the low bit-rate and the high bit rate line run in parallel (e. g. in the binder between the cabinet and the building), a Downstream Power Back-Off (DPBO) mechanism is applied on the high bit-rate telecommunication lines. DPBO allows shaping the Power Spectral Density (PSD) of a downstream signal from the modem circuitry in the cabinet to the modem circuitry of the CPE. In order to protect the low bit rate lines coming from the central office or local exchange and coexisting in the same binder, DPBO basically adjusts the downstream transmit PSD for VDSL2 lines such that the far-end crosstalk (FEXT) generated by these lines is equal to the FEXT generated by the rather low bit rate lines. Therefore, the impact from VDSL2 on ADSLx becomes equal to the impact from ADSLx itself.
In order to protect the low bit rate lines from the crosstalk caused by the high bit rate lines effectively, configuration parameters for DPBO must be set correctly. In case of incorrectly set configuration parameters, the crosstalk coming from the high bit rate lines will be abnormally high and seriously impact the performance of the low bit rate lines. DPBO is described in detail e. g. in the ITU-T recommendation G.997.1, in particular in appendix II of this recommendation.